An examination of the organization and expression of the chicken histone genes is proposed. These genes code for stable, highly conserved proteins which blind tightly to nuclear DNA in the basic chromatin subunit, and whose synthesis is closely coupled to cellular DNA replication. Therefore, the regulation of histone gene expression could be a primary step in the regulation of cell division and/or differentiation. Evidence for developmental switches in sea urchin histone gene transcription has been recently obtained. One of the goals of this project is to llok for similar processes in the chicken, and to correlate such switches with their role in cell differentiation. We have isolated several recombinant DNA clones containing chicken histone genes. Our preliminary analysis of some of these clones suggests that chicken histone genes are not arranged in the simple tandemply-repeated arrays known to exist in sea urchings and Drosphila, but rather are in one or more clusters of histone genes, where these clusters lack any simple internal symmetry. We intend to extend these preliminary analyses and to examine more histone gene recombinants in order to deduce the overall genomic organization of such clusters. Individual histone genes will be compared to assess the extent of histone gene heterogeneity in the chick. Gene-specific and flanking region-specific DNA blocks will be used to probe the transcription and processing of histone gene RNA in a varietay of cell types. Special emphasis will be placed on examining the relation of histone gene diversity to the developmentaly regulated expression of these genes. In addition, chicken histone gene DNAs will be transfected into S. cerevisiae in order to examine the expression of these genes in a relatively simple in vivo system.